Lubricants in commercial use today are prepared from a variety of natural and synthetic base stocks admixed with various additive packages and solvents depending upon their intended application. The base stocks typically include mineral oils, highly refined mineral oils, poly alpha olefins (PAO), polyalkylene glycols (PAG), phosphate esters, silicone esters, diesters and polyol esters.
Stability requirements and the accompanying need for lubricating oils with greater stability have been increasing. As engines become smaller and tighter, and engine operating temperatures go higher, the need for higher stability lubricants has increased. In addition, higher stability lubricants which retain this feature are also desired when longer drain intervals and decreased maintenance are desired, both of which result in savings.
In end uses where higher stability is desired or required, polyol esters have been commonly used due to their high thermal and oxidative stability. One of the most demanding lubricant applications in terms of thermal and oxidative requirements is oils for aircraft turbines. In aircraft turbines, where operating temperatures and exposure to oxygen are both high, it has been the industry's practice to use polyol esters.
Most lubricating oil formulations require the addition of antioxidants (also known as oxidation inhibitors). Antioxidants retard the rate at which ester base stocks (or any base stocks) deteriorate in service, which deterioration can be evidenced by the products of oxidation such as sludge and varnish-like deposits on the metal surfaces, and by viscosity and acidity growth. Such antioxidants include arylamines (e.g., dioctyl phenylamine and phenyl alphanaphthylamine), phosphosulfurized or sulfurized hydrocarbons, and hindered phenols (e.g., butylated hydroxy toluene) and the like.
Frequently replacing the lubricating oil or adding an antioxidant to suppress oxidation increases the total cost of maintaining an engine or other mechanical device. It would be most desirable to have an ester base stock which exhibits substantially enhanced thermal/oxidative stability compared to conventional ester base stocks, thus requiring less frequent replacement due to decomposition (i.e., oxidation degradation).
High Pressure Differential Scanning Calorimetry (HPDSC) has been used to evaluate the thermal/oxidative stabilities of formulated automotive lubricating oils (see J. A. Walker, W. Tsang, SAE 801383), of synthetic lubricating oils (see M. Wakakura, T. Sato, Journal of Japanese Petroleum Institute, 24 (6), pp. 383-392 (1981)) and of polyol ester derived lubricating oils (see A. Zeeman, Thermochim, Acta, 80(1984)1). In these evaluations, the time for the bulk oil to oxidize under a fixed temperature and atmosphere, which is the induction time, was measured. Longer induction times have been shown to correspond to more stable oils, to oils having higher concentrations of antioxidants, to oils having more effective antioxidants, or to oils having more stable base stocks.
The use of HPDSC as described herein provides a measure of stability through oxidative induction times. An ester base stock can be blended with a constant amount of dioctyl diphenylamine which is an antioxidant. This fixed amount of antioxidant provides a constant level of protection for any ester base stock against bulk oxidation. Thus, 0118 tested in this manner with longer induction times have greater intrinsic resistance to oxidation.
The present inventors have developed a unique ester composition and method for preparing these esters such that they have enhanced thermal/oxidative stability, higher volume resistivity, low metals, low ash and low total acid number, when compared to many conventional ester compositions.
The thermal and oxidative stability which is designed into the novel ester compositions of the present invention can permit the formulation to eliminate or reduce the level of antioxidant which must be added to a particular lubricant, thereby providing a substantial cost savings to lubricant manufacturers.
Additionally, the synthetic polyol ester base stocks according to the present invention incorporate a lesser amount of straight-chain and branched-chain monocarboxylic acids having five and six carbon atoms than conventional polyol esters used in the formulation of aircraft turbine oils, thereby markedly reducing the odor. While normal C.sub.5 acids and branched C.sub.5 acids including trimethylacetic acid are judged to provide thermal/oxidative stability, they are a marketing issue in that C.sub.5 acids are highly malodorous and any time there is saponification of the polyol ester and free C.sub.5 acid is generated, customers are acutely aware of its presence. Thus, it would be highly desirable to formulate a polyol ester base stock for use in aircraft turbine oils which has reduced amounts of C.sub.5 and C.sub.6 acids, while exhibiting satisfactory thermal and oxidative stability and still meeting the requirements of Military Specification MIL-L-23699C or MIL-L-23699D.
The chemical and physical requirements for Military Specification MIL-L-23699C and 23699D are set forth below in Table 1.
TABLE 1 (MIL-L-23699C) Requirement Limits Test Method Viscosity @ -40.degree. C. (-40.degree. F.) 13,000 cSt ASTM D 2532 Viscosity % change after 72 hr @ -40.degree. C. (-40.degree. F.) .+-.6 cSt ASTM D 2532 Viscosity @ 98.9.degree. C. (210.degree. F.) 5.0-5.5 cSt ASTM D 445 Viscosity @ 37.8.degree. C. (100.degree. F.) 25.0 cSt ASTM D 445 Flash point, minimum 246.degree. C. (475.degree. F.) ASTM D 92 Pour point, maximum -54.degree. C. (-65.degree. F.) ASTM D 97 Total Acid Number (TAN), maximum 0.5 ASTM D 664 Thermal stability and corrosivity @ 274.degree. C. (525.degree. F.) 3411 of FED-STD-791 Viscosity change, % maximum 5.0 Total acid number change, maximum 6.0 Weight of metal change, maximum 4.0 mg/cm.sup.2 Corrosion and Oxidation stability 5308 of FED-STD-791 after 72 hours at test temperature 175.degree. C. 204.degree. C. 218.degree. C. Viscosity % change -5 to 15 -5 to 25 Total acid number, change maximum 2.0 3.0 Metal weight change mg/cm.sup.2 steel .+-.0.2 .+-.0.2 .+-.0.2 silver (Ag) .+-.0.2 .+-.0.2 .+-.0.2 aluminum (Al) .+-.0.2 .+-.0.2 .+-.0.2 magnesium (Mg) .+-.0.2 .+-.0.2 -- copper (Cu) .+-.0.4 .+-.0.4 -- titanium (Ti) -- -- .+-.0.2
Generally, polyol esters used in forming aircraft turbine oils typically include a mixture of monopentaerythritol and dipentaerythritol esters. Still others have blended trimethylolpropane esters and dipentaerythritol esters, trimethylolpropane esters and monopentaerythritol/dipentaerythritol esters, or a mixture of trimethylolpropane esters and monopentaerythritol esters.
For example, U.S. Pat. No. 4,826,633 (Carr et al.), which issued on May 2, 1989, is directed to a synthetic ester base stock for use in lubricants for gas turbine engines which meet the specifications of the bearing rig test referred to in military specification ML-23699C. The synthetic ester base stock disclosed in Carr et al. is formed by reacting at least one of monopentaerythritol and trimethylolpropane with a mixture of aliphatic monocarboxylic acids, The mixture of acids includes straight-chain acids having from 5 to 10 carbon atoms and an iso-acid having from 6 to 10 carbon atoms, preferably iso-nonanoic acid. This synthetic ester base stock when mixed with a standard lubricant additive package provides a lubricant having a viscosity at 210.degree. F. (99.degree. C.) of at least about 5.0 centistokes and a pour point of at least as low as about -65.degree. F. (-54.degree. C.). Carr et al. teaches away from the use of technical grade pentaerythritol which includes 11-13 wt. % dipentaerythritol. Carr et al. argues that the dipentaerythritol contained in technical grade pentaerythritol produces an ester lubricant which exhibits increased carbonization and depositing along the oil-air-metal interface compared to that of esters formed without dipentaerythritol. Moreover, Carr et al. neither described nor suggested how much C.sub.5 acids can be used in forming the ester base stock to avoid the odor generated from the release of valeric acid and still satisfy the military specifications discussed above.
U.S. Pat. No. 4,124,513 (Yaffe) and U.S. Pat. No. 4,141,845 (Yaffe) both relate to synthetic lubricating oil compositions having improved oxidation stability. The ester base oil used to prepare the lubricant according to either Yaffe patent consists of: technical grade pentaerythritol ester made from a mixture of carboxylic acid consisting of: iso-C.sub.5, normal C.sub.5, normal C.sub.6, normal C.sub.7, normal C.sub.8 and normal C.sub.9. Therefore, the Yaffe patents disclose that up to 51 mole % of C.sub.5 and C.sub.6 acids is the preferred base stock which teaches away from that recited in this invention.
Co-pending U.S. patent application, Ser. No. 08/284,777 (Ashcraft et al.), which was filed on Aug. 2, 1994, is directed to a synthetic ester base stock for use in formulating aircraft turbine oil which comprises the reaction product of: (a) technical pentaerythritol, and (b) a mixture of C.sub.5 -C.sub.10 carboxylic acids. The mixture of C.sub.5 -C.sub.10 carboxylic acids comprises: (1) from 5 to 20 mole % of at least one C.sub.8 -C.sub.10 carboxylic acid each having 6 or less reactive hydrogens, (2) from 50 to 65 mole % of at least one C.sub.5 -C.sub.7 carboxylic acid each having 6 or less reactive hydrogens, and (3) at least 15 mole % of at least one C.sub.6 -C.sub.10 carboxylic acid each having more than 6 reactive hydrogens. The C.sub.5 -C.sub.7 carboxylic acid is preferably n-pentanoic acid or 2-methylbutanoic acid. This teaches away from the ester base stock according to the present invention which requires that the amount of C.sub.5 acid is substantially reduced to avoid the release of malodorous C.sub.5 acid.
Every lubricant has a characteristic odor which is imparted to it by the compositional changes which occur when used in an engine. In particular, if there is any decomposition of the ester component, it is expected that free carboxylic acid will be generated. In general, hydrolysis of synthetic ester base stocks containing significant amounts of lower molecular weight acid give rise to decomposition products of greater odor intensity than those containing lesser amounts of lower molecular weight acids. By lower molecular weight acids the present inventors mean pentanoic and, to a lesser extent, hexanoic acids which have five or six carbon atoms, respectively. Further, both straight-chain and branched-chain acids are included in this definition. This is true whether the lower molecular weight acids are combined with trimethylolpropane, monopentaerythritol or dipentaerythritol.
It is an object of the present invention to make a high stability synthetic lubricant base stock which provides lubricants having viscosity and pour point characteristics capable of meeting the military specifications for aircraft turbine oils, while minimizing the amount of pentanoic and hexanoic acids contained therein.
The present inventors have developed a unique synthetic ester base stock for use in aircraft turbine oil lubricants having a decreased tendency to release malodorous pentanoic and hexanoic acids when used in an aircraft turbine engine. Furthermore, the synthetic ester base stock of the present invention when combined with a standard lubricant additive package provides a lubricant which meets military specification MIL-L-23699C or MIL-L-23699D with a viscosity at 210.degree. F. (99.degree. C.) of at least 5.0 cSt and at -40.degree. C. of no more than 13,000 cSt, and a pour point of less than at least -65.degree. F. (-54.degree. C.). This ester base stock also exhibits excellent thermal and oxidative stability characteristics.
The present invention also provides many additional advantages which shall become apparent as described below.